This is the way to do it. Done well, a 5-year deep dive can make you a world-class expert in most things. Do that, and then move onto the next thing, bringing that skillset along.

It goes faster every time too. It turns out there are lots of overlaps. Linear algebra and differential equations are (roughly) the same in electronic engineering, quantum mechanics, or mechanical engineering, and where they're not is where you often bring in unique insights.

The people who struggle are the ones who go shallow. A half-dozen HN articles a day.